[Python] Python - AttributeError: 'str' object has no attribute 'append'

I keep receiving this error when I try to run this code for the line "encoded.append("i")":

AttributeError: 'str' object has no attribute 'append'

I cannot work out why the list won't append with the string. I'm sure the problem is very simple Thank you for your help.

def encode(code, msg):
'''Encrypts a message, msg, using the substitutions defined in the
dictionary, code'''
msg = list(msg)
encoded = []
for i in msg:
if i in code.keys():
i = code
encoded.append(i)
else:
encoded.append(i)
encoded = ''.join(encoded)
return encoded

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